Judging from the pictures you posted, I think you need to do the following (just opinion!):
You seem to be a pretty average-sized guy, and the good thing is you're longer than you are wide. You do have a bit of fat though, and it looks like you're not a stick, but this isn't a bad thing. The polo you're wearing in the photo would look tons better if the sleeves were more fitted. American Eagle has some decent fitted polos, and they're much much cheaper than the ones sold at Abercrombie/Hollister.
Personally, since you usually shop at Wal-Mart, I don't think you should go for Abercrombie/Hollister. For one thing, it's expensive. For another, everyone wears it. And thirdly, its a stereotyped clothing. Try some other, still popular, though not so ubiquitous stores, like the GAP, Zara, Urban Outfitters, Le Chateau, Mexx. American Eagle & Old Navy are good for cheap clothing, but again, it's pretty generic and everyone wears it. But at least it's cheap!
You look like you have a decent frame, so try to wear things that are a little more fitted, but not extremely so. You might grow out of them and then the clothing will be worthless. Buy clothing that gives you a bit of room to breathe, but still sits on your body as though it's a second skin. And don't just go for your go-to sizes. Zara often has rather small clothing (you might very well be an extra-large) while the GAP tends to have rather large clothing (you might be a medium to a small). Try on everything you see.
And go for the sales! If it's not a good time to buy, then wait. Here in Toronto, everyone was having a pretty big spring to summer season sale, but that may very well be over down in Texas. Clothing is expensive, and there's no reason to spend an exorbitant amount of money on it. Besides that, if you usually shop at Wal-Mart, I doubt your parents are going to let you start shopping at full-price Abercrombie/Hollister.
Let us know how it goes!